1. Field of the Invention
This invention relates to an organic light-emitting diode, for a display, with low cathode contact resistance, and a method for fabricating the same.
2. Description of the Related Art
Displays using an organic light-emitting diode (OLED) provide great brightness and a wide viewing angle. OLEDs are self-luminous so they do not require back light and can be effectively used even under relatively dark ambient conditions.
OLEDs include an organic semiconductor layer which acts as an emissive layer between two electrodes. At least one of these electrodes is formed of transparent material.
Generally, a transparent anode is formed by coating, for example, indium tin oxide (ITO), which is transparent to visible ranges of light waves, on a glass substrate. A cathode is formed, for example, of a metal, such as aluminum, by vapor deposition.
When a voltage is applied across the electrodes, colored light may be emitted, wherein the color of light emitted varies depending on the material of the organic emissive layer. For light emission, electrons and carriers (holes) are injected from the cathode and the anode, respectively, and the electrons and carriers migrate into the organic emissive layer under an electric field to combine in the organic emissive layer. Exitons, which are electrically neutral, are generated by the combination of the electrons and the holes in the organic emissive layer. As this excited state transits to a base state, light is generated.
EP 1083612 A2 discloses an OLED with a cathode having a multi-layered structure, such as, for example, lithium fluoride, calcium, and aluminum layers or barium and silver layers. Lithium, calcium, and/or barium layers act to inject electrons into an emissive layer. A lithium fluoride layer has a thickness, for example, of a few nanometers, and a barium and/or calcium layer has a thickness, for example, of 100 nm. An aluminum or silver layer transports a large amount of electrons from a cathode contact layer to the emissive layer. The aluminum or silver layer has a thickness, for example, of 0.2–2 μm. Thus, in such a multi-layered cathode, the lithium chloride, calcium, or barium layer forms an electron injecting layer, and the aluminum or silver layer forms an electrical conducting layer of the cathode.
The cathode of an OLED is electrically connected to an electrical driving system of a display via a flexible printed circuit (FPC). FPCs are used in manufacturing LCD displays, as well as, OLED-based displays. An FPC is bound to a display substrate by a hot sealing method using an anisotropic adhesive film under predetermined temperature and pressure conditions. When the FPC is directly bound to the cathode by hot sealing, the cathode may be damaged by the pressure and the temperature applied to seal and bind the FPC to the cathode. Accordingly, the FPC and the cathode (including an electron injecting layer and an electrical conducting layer) should be separated from one another. A cathode contact layer may be used to separate the cathode and the FPC from one another. The cathode contact layer may be formed of, for example, ITO. For electrical contact between the cathode and the cathode contact layer, the cathode is formed to extend over the emissive layer and a portion of the cathode contact layer by thermal evaporation using, for example, a shadow mask. A portion of the cathode contact layer which is not in contact with the cathode is used to connect with the FPC or other parts of a display. With such an arrangement, the cathode and the FPC can be electrically connected without direct contact therebetween. Accordingly, in such an arrangement, charges flow between the cathode and the FPC via the cathode contact layer.
FIG. 1 is a sectional view of a conventional OLED having the structure described above. In FIG. 1, an anode layer 2 is arranged on a substrate 1, and an emissive layer 3, which represents a pixel surface during display operation, is arranged on the anode layer 2. A cathode contact layer 7 is arranged on the substrate 1 and, as shown in FIG. 1, the cathode contact layer 7 is not in direct contact with the anode layer 2. An electron injecting layer 4 which is formed, for example, of lithium chloride, is arranged on the emissive layer 3 and a portion of the cathode contact layer 7. An electrical conducting layer 5 formed for example, of aluminum, is arranged on the electron injecting layer 4. An FPC 6 is arranged on a portion of the cathode contact layer 7 that is not covered with the electron injecting layer 4 and the electrical conducting layer 5. In this way, a cathode comprising the electron injecting layer 4 and the electrical conducting layer 5, can be electrically connected with the FPC 6 without direct physical contact.
In a display including a number of pixels arranged in a matrix, as is well known, a cathode of the display comprises a plurality of cathode lines and each cathode line is commonly connected to the plurality of pixels. However, this cathode structure disadvantageously increases electrical resistance gradually as a current flows along the cathode lines.
To resolve this problem, JP 10294183, for example, discloses the use of an auxiliary cathode formed of a high-current conductive material. The same approach for an anode is found, for example, in JP 2001015268. JP 2001282136 discloses the use of an aluminum auxiliary layer underneath an ITO cathode contact layer to prevent ohmic loss in a cathode contact layer and cathode lines.
However, in a multi-layered cathode including an electron injecting layer and an electrical conducting layer, calcium, barium, and lithium chloride layers, etc., which are used to form the electron injecting layer, are known to increase electrical resistance. In addition, the calcium or barium layer, for example, is partially oxidized through a reaction with oxygen present on the substrate or in an evaporation system, into calcium oxide or barium oxide, which are electrical insulators. As a result, the electrical contact resistance between the cathode and the cathode contact layer increases. A greater electrical resistance caused by the cathode requires a larger amount of power to operate an OLED or a display employing such a cathode, so that undesirable heat development occurs in the OLED or the display.